1. Field of the Invention
The invention relates to a sewing machine with an automatic thread cutter for carrying out a thread cutting operation after reducing or nullifying a feed pitch of a work fabric.
2. Description of the Related Art
A conventional sewing machine with an automatic thread cutter is provided with a thread terminal holding device for holding a terminal portion of a needle thread carried by a needle over a machine bed and releasing the thread terminal portion at the start of the next sewing, so as to shorten a residual length of the thread terminal portion upon cutting of the needle thread in association with the end of sewing. In the case of a sewing machine that is not provided with the above thread terminal portion holding device, there is a possibility that a stitch is not formed (skip stitch) or the needle thread is taken out of an eyelet of the needle because of the enlargement of a needle thread loop by a rotating hook or lifting of a thread take-up lever at starting of the sewing (first stitch), which causes an increase in the residual length of the thread terminal portion after thread cutting. This increase in residual length greatly damages the appearance of a sewn fabric, and it is troublesome in that an operator must later cut the thread terminal portion with scissors or the like.
In the sewing machine with the thread terminal portion holding device, the needle thread held by the thread terminal portion holding device can be released at an arbitrary timing in association with the start of sewing. Therefore, the skip stitch or the release of the needle thread from the eyelet of the needle can be prevented to thereby shorten the residual length of the thread terminal portion.
The shorter the residual length of the thread terminal portion, the shorter the length of the needle thread extending from the sewn fabric. Accordingly, it is not necessary to cut the thread terminal portion later, and catching of the thread terminal portion by a second stitch or subsequent stitches is prevented.
In the conventional sewing machine with the automatic thread cutter, a feed pitch of the work fabric is reduced or made zero upon thread cutting to further shorten the residual length of the thread terminal portion. This is due to the following reason.
Thread cutting is performed during a period of time from the time when the needle comes out of the work fabric to the time when the needle reaches a top dead center of the stitching stroke. On the other hand, feeding of the work fabric is also performed during a period of time while the needle is separate from the work fabric. Accordingly, in order to prevent the work fabric from being fed a great distance during a period of time until the thread cutting is actually performed, and the needle thread being drawn from the eyelet of the needle to lengthen the thread terminal portion, the feed pitch of the work fabric is reduced or made zero upon initiation of the thread cutting operation.
FIG. 5 shows a timing chart of the feed pitch during the thread cutting operation.
When a foot pedal is reverse depressed, that is a heel portion of the pedal is depressed, by an operator to start the thread cutting operation at the end of sewing of the work fabric, a thread cutting command signal is input to shift a rotational speed of the machine motor from a high speed to a low speed. After the low speed has been reached, a needle position signal (a detection signal at a bottom dead center position of the needle in this timing chart) is detected. According to this detection signal, a work fabric feed pitch control start signal for reducing the feed pitch of the work fabric is output. After the work fabric is fed at the reduced pitch, the thread cutting is started according to a thread cutting operation signal. At the time the thread cutting is ended, the feeding of the work fabric by the machine motor is stopped.
Thus, in the conventional sewing machine with such an automatic thread cutter, after the rotational speed of the machine motor has completely shifted over to the low speed, then the needle position signal is detected to carry out the thread cutting operation. As a result, a considerable period of time is required from the time when the thread cutting command signal is input to the time when the thread cutting operation is ended, thus reducing the work rate of the operator.